If it's not the case, you can try adjusting your monitor brightness and contrast settings.
Also, if the grayscale has a color tint and your monitor has a color temperature setting you can try to play with it.
Note that if you have a light source in your room this will influence your color perception. In such a case, it's better
to adjust your settings in the dark or in daylight.

For an in-depth explanation of monitor calibration theory, you may want to refer to the following articles :